Depending on the type of cancer a dog is diagnosed with, we are either focused on controlling the tumor locally or preventing/delaying metastasis (spread of cancer cells to another organ) or both.
Achieving local control means that we have successfully removed/killed all cancer cells at the primary tumor site. The primary tumor site is the location where cancer started (lung lobe, skin mass, bone, etc.). This is typically accomplished using surgery or a combination of surgery plus radiation or electrochemotherapy.
In cases where a tumor is either malignant and has a high risk of metastasis or has already metastasized, chemotherapy is indicated to delay metastasis, which ultimately helps the patient live longer. Sometimes this helps the patient live for years, sometimes 6-12 months. This varies significantly based on the type of tumor we are treating and how aggressive that particular patient's tumor is.
So, how do we know how successful our treatments will be? How do we know if cancer will return after surgery or if cancer will spread to another organ (like the lungs, liver, a lymph node, etc.)? How do we know if we should bother with chemotherapy or not? If we choose chemotherapy, how do we know if it's working? If it's working initially, why does it stop working? Continue reading to find out...
How do we know how successful our treatments will be?
When we treat a dog with a particular type of chemotherapy for a particular type of cancer, we can estimate how long they will live as a result of receiving that treatment based on studies that have been published.
The studies typically tell us either the median survival or the median progression free interval. If the median survival time is 300 days, that means that half of the dogs lived longer than 300 days and half didn’t live as long in one particular study; it’s similar to an average. The median progression free interval tells us the median number of days that a patient is expected to respond to a treatment (remembering that half will do better and half not as well).
How do we know if cancer will return after surgery or if cancer will spread to another organ?
For many types of cancer, studies have been published that tell us the recurrence rate (the likelihood of the tumor growing back at the surgery site) and the metastatic rate (the likelihood of cancer spreading to another organ).
Sometimes the grade of the tumor influences the chance of recurrence after surgery; this is the case for mast cell tumors and soft tissue sarcomas. The higher the grade of the tumor, the greater the chance of recurrence.
The greatest factor influencing whether a skin tumor will recur after surgery is whether the surgery was able to achieve "clean margins". Whenever surgery is performed, the surgeon has to remove the tumor plus a rim of normal tissue surrounding the tumor and below the tumor. This is because cancer cells are scattered in these tissues, we just cannot see them. For certain types of tumors, removing the tumor plus 1 cm of normal-appearing tissue is appropriate. For other types of tumors, removing the tumor plus 3 cm of normal-appearing tissue plus a "fascial plane deep" (a very fibrous layer of fascia that is adherent to the underlying muscle layer) is needed to achieve clean margins.
If enough tissue is not removed the excision might be incomplete, in which case we may expect a new tumor to regrow over the next few months. In other cases the excision might be "narrow" (ie narrow margins), in which case a second surgery or radiation should be performed in many cases if we don't want the tumor to recur.
An oncologist can help interpret the pathology report (the report that is received after surgery to help determine a diagnosis and also to help determine how "successful" surgery was) and provide recommendations.
Every type of cancer has a certain “pattern of metastasis”, meaning certain organs that it is more likely to spread to than others. By knowing this, it helps us determine if cancer has already metastasized before treatment (such as surgery) is pursued.
Typically, if a patient has a cancer with a metastatic rate of more than 25% or so (greater than 25% risk of spread to another organ), chemotherapy will be recommended to help that patient live longer.
How do we know if we should bother with chemotherapy or not?
Whether to give chemotherapy is a personal choice. An oncologist can discuss various chemotherapy options with you, what each would be like for you and for your dog, as well as how long your dog would be expected to live with each option.
Some people only elect chemotherapy if it will help their dog live for years. Others will elect chemotherapy if it will help their dog live for 6-12 months, everyone is different.
Remember that (in the right hands) chemotherapy is typically well tolerated. Our goal with chemotherapy is to help your dog live longer, but not to compromise quality of life.
If your dog doesn't feel well with one treatment, things can be adjusted (give anti-nausea or anti-diarrhea medications prophylactically, reduce the dose of chemotherapy, etc.).
If we choose chemotherapy, how do we know if it's working?
If we are giving chemotherapy to a patient with microscopic cancer cells, then the only way we know it's "working" is if we don't find any evidence of cancer on "staging tests".
For example, a common treatment for osteosarcoma is to remove the affected leg to resolve the patient's pain, then treat with chemotherapy to delay the spread of cancer cells to the lungs.
To ensure that there is no evidence of metastasis, we take chest x-rays (thoracic radiographs) when the patient comes for the 3rd and 6th (final) chemotherapy treatments. If we were to find evidence of metastasis on the x-rays, we would not give those treatments, because metastasis means that chemo is not working. Alternatively, we would discuss a plan B. If the patient staged clean (no evidence of cancer in lungs on those visits), we would finish chemo as planned and monitor for cancer recurrence with additional x-rays every 3 months for the first year and (typically) every 6 months thereafter.
If we are treating a patient that has a measurable tumor that we can see or feel on exam (or on chest x-ray or abdominal ultrasound), then we measure that tumor as we proceed with treatment. Based on the size of the mass (and a few other factors), we can determine if that patient is responding. Typically, if disease is stable or improved, we continue with our chemotherapy protocol. If disease is progressive (worse) we need to change the chemotherapy protocol.
If chemotherapy is working initially, why does it stop working?
As we treat cancer, it changes. Cancer cells can pick up new mutations, which make them resistant to the chemotherapy treatments they initially responded to. There are actually many ways in which cancer cells develop resistance - many people have spent their entire careers studying this. It is a great challenge of ours when treating diseases that rely on chemotherapy, such as lymphoma.
Chemotherapy can be scary as a dog owner, but it can also be life-saving. If you're wondering if chemo can help your dog, consider a consultation with your nearest oncologist.
Have questions about this article? Reach out!
Dr. Lori Cesario
Board Certified Veterinary Oncologist